1. Field of the Invention
The present invention relates to a polishing method and polishing apparatus, a more particularly relates to a polishing method and polishing apparatus for flattening an uneven surface formed along with formation of copper interconnections in the process of production of a semiconductor device having copper interconnections.
2. Description of the Related Art
Along with the increase in integration and reduction of size of semiconductor devices, progress has been made in miniaturization of interconnections, reduction of interconnection pitch, and superposition of interconnections. The importance of the multilayer interconnection technology in the manufacturing process of semiconductor devices is therefore rising.
To form the interconnections of a semiconductor device of a multilayer interconnection structure, frequent use has been made of the method of forming the interconnections by aluminum or an alloy of the same, covering them by an insulating film, then burying contact holes passing through this insulating film by tungsten or an alloy of the same to connect with the lower interconnections.
With the above method, step differences arise due to the interconnections on the surface of the covering insulating film after the formation of the interconnections. Along with the miniaturization of interconnections, since the depth of focus at the photolithography of the upper layer can no longer be sufficiently matched with, a need has arisen for flattening these step differences. The method has been employed of polishing the insulating film by chemical mechanical polishing (CMP) method.
Further, use has been made of the metal CMP method of burying through holes formed in the insulating film by the above tungsten or other metal, removing the excess metal film by CMP, and thereby connecting with the lower interconnections.
On the other hand, in order to suppress the propagation delay of signals in the recent 0.25 μm or less design rule, an interconnection process for replacing the aluminum of the interconnection material by copper is being developed. When using copper for interconnections, there is the merit that both a low resistance and a high electromigration tolerance can be obtained.
In a process using this copper for interconnections, for example, an interconnection process referred to as the damascene process for burying a metal in groove-like interconnection patterns formed in an interlayer insulating film in advance and removing excess metal film by CMP to form the interconnections has become dominant.
The damascene process has the characteristics that etching of the interconnections become unnecessary and also a further upper interlayer insulating film becomes flat by itself, so the process can be simplified.
Further, by the dual damascene process where not only grooves for the interconnections, but also the contact holes are formed as grooves in the interlayer insulating film and where the interconnections and the contact holes are simultaneously buried by the metal, a greater reduction of the interconnection steps becomes possible.
Here, an explanation will be made of an example of the process for forming copper interconnections by the dual damascene process by referring to the figures below.
First, as shown in FIG. 11A, for example, an interlayer insulating film 302 made of for example silicon oxide is formed by for example low pressure chemical vapor deposition (CVD) on a silicon or other semiconductor substrate 301 on which a not illustrated impurity diffusion region is appropriately formed.
Next, as shown in FIG. 11B, contact holes CH communicating with the impurity diffusion region of the semiconductor substrate 301 and grooves M in which will be formed a predetermined pattern of interconnections to be electrically connected to the impurity diffusion region of the substrate 301 are formed by using well-known photolithography and etching.
Next, as shown in FIG. 11C, a barrier metal film 305 is formed on the surface of the interlayer insulating film 302 and in the contact holes CH and the grooves M. This barrier metal film 305 is formed by a material such as Ta, Ti, TaN, or TiN by well-known sputtering. When the interconnection material is copper and the interlayer insulating film 302 is silicon oxide, since copper has a large diffusion coefficient with respect to silicon oxide, it is easily oxidized. The barrier metal film 305 is provided to prevent this.
Next, as shown in FIG. 11D, a seed film 306 is formed on the barrier metal film 305 by depositing copper to a predetermined thickness by well-known sputtering.
Then, as shown in FIG. 11E, a copper film 307 is grown and formed on the seed film 306 so as to bury the contact holes CH and the grooves M by copper. The copper film 307 is formed by for example plating, CVD, sputtering, etc.
Next, as shown in FIG. 11F, the excess copper film 307 and barrier metal film 305 on the interlayer insulating film 302 are removed by CMP for flattening.
Due to the above steps, copper interconnections 308 and contacts 309 are formed.
By repeating the above process on the interconnections 308, multilayer interconnections can be formed.
Summarizing the problems to be solved by the invention, in the step of removing the excess copper film 307 by CMP in the copper interconnection forming process using the dual damascene process, because the flattening technique employing conventional CMP involves applying a predetermined pressure between a polishing tool and the copper film for polishing, there arises a problem that large damage is given to the semiconductor substrate. Especially, in a case where an insulating film of a small dielectric constant having a low mechanical strength, which will be important in the 0.13 μm generation and on, is used, the aforesaid damage is no longer negligible and may cause cracks of the interlayer insulating film and separation of the interlayer insulating film from the semiconductor substrate.
Further, the removal performance differs among the interlayer insulating film 302, the copper film 307, and the barrier metal film 305, therefore there has been the problem that dishing, erosion (thinning), recess, etc. easily occur in the interconnections 308.
Dishing is a phenomenon where, as shown in FIG. 12, when there is an interconnection 308 having a width of, for example, about 100 μm at a 0.18 μm design rule, the center portion of the interconnection is excessively removed and sinks. If this dishing occurs, the sectional area of the interconnection 308 becomes insufficient. This causes poor interconnection resistance etc. This dishing is apt to occur when copper or aluminum, which are relatively soft, is used as the interconnection material.
Erosion is a phenomenon where, as shown in FIG. 13, a portion having a high pattern density such as where interconnections with a width of 1.0 μm are formed at a density of 50% in a range of for example 3000 μm is excessively removed. When erosion occurs, the sectional area of the interconnections becomes insufficient. This causes poor interconnection resistance etc.
Recess is a phenomenon where, as shown in FIG. 14, the interconnection 308 becomes lower in level at the interface between the interlayer insulating film 302 and the interconnection 308 resulting in a step difference. In this case as well, the sectional area of the interconnection becomes insufficient, causing poor interconnection resistance etc.
Further, in the step of flattening and removing the excess copper film 307 by CMP, it is necessary to efficiently remove the copper film. The amount removed per unit time, that is, the polishing rate, is required to be for example more than 500 nm/min.
In order to obtain this polishing rate, it is necessary to increase the polishing pressure on the wafer. When the polishing pressure is raised, as shown in FIG. 15, a scratch SC and chemical damage CD are apt to occur in the interconnection surface. In particular, they easily occur in soft copper. For this reason, they cause opening of the interconnections, short-circuiting, poor interconnection resistance, and other defects. Further, if the polishing pressure is raised, there is the inconvenience that the amount of the scratches, separation of interlayer insulating film, dishing, erosion, and recesses also becomes larger.